In a cryogenic air separation method wherein nitrogen, oxygen, argon and the like are separated from air by distillation, it is necessary to perform a pre-step wherein water, carbon dioxide, and other trace impurities (hydrocarbons, nitrogen oxide and the like) are removed from feed air in advance before the feed air is cooled at a cryogenic temperature to perform distillation. Said pre-step is called pre-purification of air.
When air which includes said impurities is cooled to a cryogenic temperature as it is, for example, there is a possibility that water, carbon dioxide and nitrogen oxide become solidified and choke piping and the like, or that hydrocarbons concentrate in a liquid oxygen tank and become a factor which inhibits safety operations.
An adsorption method has already been used for pre-purification of a cryogenic air separation method in general.
A temperature swing adsorption method (TSA method) has been used as one of such pre-purification methods wherein adsorption is used. This method is performed with a pre-purification apparatus having plural adsorption columns. In the adsorption columns, an adsorbent (such as activated alumina, silica gel, K-A type zeolite, Na-A type zeolite and the like) is filled to remove water at the inlet side to which feed air enters, and another adsorbent (such as a Na—X type zeolite and the like) is filled at the downstream side of the column to remove carbon dioxide.
The TSA method purifies air in succession with said pre-purification apparatus by alternatively performing an adsorbing step wherein impurities included in feed air are adsorbed by adsorbents at a relatively low temperature, and a regenerating step wherein the adsorbents are regenerated at a relatively high temperature.
A pressure swing adsorption method (PSA method) is cited as another pre-purification method. Similar to the TSA method, this method uses a pre-purification apparatus having plural adsorption columns to perform purification. The PSA method purifies air in succession by alternatively performing an adsorbing step wherein impurities included in feed air are adsorbed by adsorbents at a relatively high pressure, and a regenerating step wherein the adsorbents are regenerated at a relatively low pressure. Similar to the TSA method, in the PSA method, an adsorbent (such as activated alumina, silica gel, K-A type zeolite, Na-A type zeolite and the like) is filled to remove water at the inlet side of an adsorption column where air enters, and another adsorbent (such as a Na—X type zeolite or the like) which selectively removes carbon dioxide is filled at the downstream side of the former adsorbent.
A conventional TSA method generally includes the following steps;
(a) an adsorption step wherein pressurized air is introduced into an adsorption column to remove impurities from the introduced air;
(b) a decompression step wherein the interior of the adsorption column is reduced to atmospheric pressure, after the adsorption step is terminated;
(c) a heating step wherein a purge gas, which does not comprise impurities, is heated and introduced in the adsorption column to perform thermal-regeneration of adsorbents, after the decompression step is terminated;
(d) a cooling step wherein a purge gas, which does not comprise impurities and is not heated, is introduced in the adsorption column to cool the interior of the adsorption column at a temperature wherein the adsorption operation is performable, after the heating step is terminated; and(e) a pressurization step wherein pressurization is performed for the cooled adsorption column by purified air, after the cooling step is terminated.
As described above, adsorbents which are suitable for a component to be adsorbed are used when impurities are removed from air by introducing pressurized air into the adsorption column. Adsorbents have characteristics wherein the adsorption capacity thereof increases at a high operating pressure and at a low operating temperature. Therefore, it is preferable that air to be treated in an adsorption column have a high pressure and a low temperature.
In order to provide such preferred conditions, feed air is conventionally compressed to a predetermined pressure by an air compressor, then cooled to about 40° C. by an after-cooler, and subsequently further cooled to 5 to 15° C. using a refrigerating machine, before feed air is supplied to an adsorption column.
However, an eco-friendly apparatus has been expected in recent years, and therefore, a method has been proposed in Japanese Patent No. 3416391, wherein pre-purification is performed without a refrigerator (freon refrigerator) which uses freon.
Air purification performed under conditions wherein a refrigerating machine is not used has a problem. That is, it is necessary to treat a large amount of water included in air. The higher a temperature is, the larger the amount of saturated water in the air is. Accordingly, when a refrigerating machine is not used and air which is saturated with water is not sufficiently cooled by an after-cooler, a large amount of water enters an adsorption column. Furthermore, due to adsorption heat which is generated when an adsorbent adsorbs water, the entered air is further heated, and another adsorbent which exists at the downstream side is required to absorb carbon dioxide at a high temperature of 60° C. or more.
The invention described in U.S. Pat. No. 3,416,391 solves such unfavorable conditions by the following points (1) to (3).
(1) Decrease of a load by reducing a cycle time
(2) Selection of an absorbent which absorbs a large amount of carbon dioxide
(3) Optimization of an amount of a regeneration gas and a heating temperature at the time of regeneration
Here, in the pre-purification of feed air using the TSA method, a large amount of a purge gas is required in the heating step (c), wherein thermal-regeneration of an adsorbent is performed, and the cooling step (d) wherein the adsorbent is cooled.
As a purge gas, clean air is desirable. In a cryogenic air separation method, a non-product gas (referred to as waste nitrogen or a waste gas) produced in a distillation step, which is performed to generate product gases (nitrogen, oxygen, argon and the like), has been generally used as a purge gas which is used in a cryogenic air separation plant. The reason is that use of such a waste gas enables a decrease in costs as compared with a case wherein air from which impurities such as water, carbon dioxide and the like are removed is produced separately.
The technique according to the invention described in the aforementioned U.S. Pat. No. 3,416,391 enables the regeneration of adsorbents without a cooling step wherein cooling is performed at 5 to 15° C. by a refrigerating machine. In the technique, regeneration of adsorbents can be performed such that a regeneration gas ratio (a regeneration gas ratio=a flow rate of a purge gas used in pre-purification/a flow rate of feed air used in pre-purification) is about 45% at a heating temperature of 200° C., under conditions where air pressure is 700 KPa (Absolute), an air temperature which flows into an adsorption column is 40° C. and a cycle time is 2 hours.
On the other hand, regarding product yield, it has been reported that product yield which can be achieved by the present technical level is about 60%, from the viewpoint of distillation ability of a cryogenic air separation method wherein a single column is used to merely generate nitrogen as a product. Furthermore, regarding a cryogenic air separation method which is a double column process type method and can generate nitrogen and oxygen as products, it has been reported that product yield thereof is about 80%.
However, it should be understood that the aforementioned yields are rough estimates, since product yield in a cryogenic air separation method depends on conditions such as operation pressure and total processes of the cryogenic air separation method.
A regeneration gas ratio of the aforementioned pre-purification method wherein no refrigerating machine is used is 45%. The smaller the regeneration gas ratio is, the larger the manufacturing yield in a cryogenic air separation method is. Pre-purification is an essential step for a cryogenic air separation method. Therefore, the amount of a product, that is, a total amount of a product generated by a cryogenic air separation plant is the remainder, which is obtained by reducing the amount of a regeneration gas required for pre-purification (the amount of a purge gas used for pre-purification) from the amount of feed air. For example, when the amount of a gas used for regeneration is 45% and the amount of feed air is 100%, the amount of a product is equal to or less than a value which is obtained by subtracting 45%, which is the amount of a gas required for regeneration, from 100%, which is the amount of feed air. That is, the amount of a product is 55% or less.
Accordingly, as the case stands, current performances of pre-purification limit a total increase of a production performance of a cryogenic air separation method.
In recent years, due to the increase of distillation performances achieved by the process improvement in a cryogenic air separation method, an increase of a yield of a product gas obtained by separation is expected. However, it is still necessary to use a large amount of a purge gas in pre-purification, and therefore a constant amount of a waste gas generated in a distillation step is required for pre-purification. Accordingly, there is a problem in that a sufficient product yield which corresponds to the improved distillation performances cannot be achieved.
In addition, a method has been disclosed in the invention described in Japanese Unexamined Patent Application, First Publication No. 60-139311, in which raw air (air which includes water and carbon dioxide to some extent) is used as a purge gas. In the invention, it is reported that “ambient air (raw air) is supplied singly or in combination with a nitrogen gas to a part of, or all parts of, a beating and temperature-rising step as a regeneration gas for an absorbent”. Furthermore, in FIG. 3 showing Example thereof, it is shown that a regeneration gas (purge gas) is supplied from the end of a purified gas outlet of an adsorption column.
However, a high-performance carbon dioxide adsorbent, which has been used in recent years, has a carbon dioxide adsorbing ability which remarkably deteriorates when the adsorbent adsorbs moisture. Accordingly, it has been believed that such a method, wherein raw air which includes water and carbon dioxide to some extent is used as a part of, or all of, a purge gas as disclosed in Japanese Unexamined Patent Application, First Publication No. 60-139311, cannot be adopted in a case wherein a high-performance carbon dioxide adsorbent is used.